<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        * {
            margin: 0;
            padding: 0;
        }
        
        .box {
            width: 100px;
            height: 100px;
            border-radius: 50px;
            background-color: blue;
        }
        
        .box:hover {
            /* 慢慢增长 */
            width: 300px;
            /* transition属性 逐渐增长 */
            transition: width 0.5s;
        }
    </style>
</head>

<body>
    <div class="box"></div>
</body>

</html>